1. Field of the Invention
The present invention relates to sheet transport control in a printing apparatus such as a copier or a printer, and more particularly to transportation of tab sheets.
2. Description of the Related Art
Apparatuses having a function of inserting tab sheets at predetermined positions, such as between chapters of a publication, are well known among recently introduced image-forming apparatuses such as copiers and printers. Because of the tab or tabs attached to a tab sheet, the size of a tab sheet cannot be found using the methods commonly used to find the size of typical rectangular sheets. In sheet transport control, movement of the transported sheet is detected by sensors provided at specific points along a paper path so as to enable control driving of motors and other mechanisms provided in the apparatus according to the detected movement. For sheets with a tab, the timing of detection signals from the sensors vary according to whether or not a tab portion passes through the sensors. Accordingly, when such sheets are used, there is a problem that appropriate timing control of motors and other mechanisms installed along the paper path may be impossible.
In an attempt to deal with the above problem, Japanese Patent Laid-Open Publication No. Hei 8-245047, for example, discloses an apparatus in which, by limiting sheets with tab to a specific orientation such that the tabs are located away from the paper transport direction when the tab sheets are loaded in a paper tray, in other words, by ensuring that the leading edge of all sheets in the paper transport direction are tab-free, it is possible to reliably determine the feed timing of the sheet. Alternatively, Japanese Patent Laid-Open Publication No. Hei 10-67458 discloses that, in order to circumvent feeding of tab sheets into a paper path in the main body of a copier, a specific paper tray dedicated to tab sheets and a tab printer are provided to a finisher for post-processing, such as a finisher for stapling of printed pages.
However, because with either of the above-described arts paper orientation for tab sheets loaded in the paper tray is limited, users must insert the tab sheets in the paper tray in the specified orientation, even when this operation is time-consuming or complicated. Further, the layout of a stapler and/or a puncher relative to the sheet (e.g. whether the stapler and/or the puncher are placed frontward or sideward in the paper transport direction) differs among finishers. If paper orientation must be limited when the tab sheets are loaded in the paper tray, selection of finishers will be limited to only those having an orientation corresponding to the required paper orientation.
In an apparatus disclosed in the above Japanese Patent Laid-Open Publication No. Hei 8-245047, when the tab sheet is inverted for duplex printing, the orientation of the tab is reversed from its position during upper surface printing. Because the reversed orientation therefore makes proper timing of transport control impossible, the apparatus has a problem in that tab sheets cannot be duplex printed.
Because the tab printing mechanism provided on the finisher does not support duplex printing, the apparatus disclosed in Japanese Patent Laid-Open Publication No. Hei 10-67458 is incapable of performing duplex printing on tabbed sheets. Because a finisher modified to support duplex printing becomes very large, such modification is not practical. Further, because the described printing mechanism which may be installed on the finisher is capable of producing printed output only on small regions such as tabs, it is impossible to produce printed output on the main body of the tab sheets.
The present invention, which was conceived in view of the aforesaid current problems, aims to provide a sheet transport apparatus in which tabbed sheets can be loaded in a paper tray in an arbitrary orientation and wherein appropriate timing control of such tabbed sheets is possible, even when duplex printing is executed.
An apparatus for sheet transport control according to the present invention, which controls sheet transport on a paper path, comprises a tab information detector for obtaining information about whether or not a tab lies on at least either one of the leading edge and the trailing edge of the sheet in a sheet transport direction and about a tab location of the detected tab on the leading edge and/or the trailing edge in a direction perpendicular to the paper transport direction when the tab is detected, a sheet traverse detector which is installed on a checkpoint established on the paper path for detecting the timing of paper traverse (the state of movement or feed of paper as it travels through the device) that at least either one of the leading edge and the trailing edge of transported sheet passes through the checkpoint, and a controller for determining operation timing of control targets according to the traverse timing detected by the sheet traverse detector. The controller determines whether or not a tab has passed through the checkpoint based on the presence or absence of the tab and relationship between the tab position obtained at the tab information detector and the position of the checkpoint in the direction perpendicular to the paper transport direction, and then adjusts the operation timing of the control targets according to whether or not the tab passes through the checkpoint.
In one aspect of the present invention, a tab information detector comprising a plurality of paper sensors, each of which is provided on either one of a plurality of positions established upstream from the checkpoint on the paper path in the direction perpendicular to the sheet transport direction, for detecting whether or not a sheet is present at the position of the sensor, detects whether or not a tab is present and the tab location in the direction perpendicular to the paper transport direction based on detection of output signals of the paper sensors.
In another aspect of the present invention, the apparatus for sheet transport control comprises a tab detector which detects whether or not a tab is present on the sheet and, when a tab is detected, the tab location, and a controller having a plurality of control modes for sheet transport in accordance with the presence or absence of the tab and the tab location on the sheet, the controller controlling sheet transport according to a control mode for sheet transport corresponding to the presence or absence of the tab and the location of the tab detected by the tab detector.
In still another aspect of the present invention, the apparatus for sheet transport control including an input interface of tab orientation which accepts input regarding which side of the sheet has a tab and a controller having a plurality of control modes for sheet transport in accordance with the side of the sheet having the tab, the controller performs sheet transport in the control mode for sheet transport corresponding to the input accepted in the tab orientation input interface.
In still another aspect of the present invention, the apparatus for sheet transport control, which controls transport of a sheet with tab, comprises a tab location detector for obtaining information about a tab location in the direction perpendicular to the sheet transport direction, a sheet traverse detector provided on a paper path for detecting at least either one of the leading edge and the trailing edge of the transported sheet so as to find traverse of the transported sheet, and a setting apparatus for establishing the transport control timing necessary for transporting the sheet based on relationship between the tab location obtained by the tab location detector and the position of the sheet traverse detector in the direction perpendicular to the sheet transport direction.